The Swedish Patent Specification Number 8204814-3, Publication Number 375 395, describes and illustrates an alarm device which incorporates detecting means in the form of electric conductors which are intended to co-act with a material whose conductivity changes when absorbing moisture, wherewith a change in conductivity between the conductors results in a change in electrical resistance or in the transition of the device from a normal, or passive, state to an active alarm state, thereby producing an alarm signal. The alarm device includes one or more trigger circuits, to which the outputs of respective associated conductors are connected.
Thus, it is known that in a normal passive state of the alarm device a pre-determined resistance and/or impedance shall prevail between associated conductors. It is also known that a given lower resistance and/or impedance shall prevail between associated conductors in the active alarm state of the device.
It is also known to incorporate in the detecting device a control circuit which, under the influence of pulses, brings the trigger circuits to an operational state in which they will react in response to said pre-determined prevailing impedance values and initiate an alarm. It is also known for the length of the pulse produced by the control circuit to be shorter than the time taken for the alarm device to react from the moment of activation to the moment at which the alarm is produced.
A number of mutually different detectors for detecting devices of this kind and for similar devices are known the art.
One example of these known detectors is found in U.S. Patent No. 2 049 321.
This U.S. Patent teaches an alarm device which incorporates a first conductor and a second conductor, said conductors being bared, or stripped, of insulation, at each detection location, so that a plate can be urged by means of a spring against a formed loop.
This U.S. specification teaches the possibility of triggering a part with the aid of moisture, such as water.
The U.S. Pat. No. 3 662 367 teaches the use of twined or plied conductors, of which one of the conductors is a plastic tube and the other is a paperinsulated electric conductor.
The platic tube is provided at pre-determined locations therealong with windows which permit moisture to pass through and wet the insulation.